Flexography is one method of printing words and images onto foil, plastic film, corrugated board, paper, paperboard, cellophane, or even fabric. In fact, since the flexographic process can be used to print on such a wide variety of materials, it is often the best graphic arts reproduction process for package printing.
The anilox cylinder serves as the heart of the flexographic press. The use of an ink-metering anilox cylinder, which is engraved with a cell pattern, enables an even and fast ink transfer to the printing plate. The configuration of the cells in the anilox roller, the pressure between the rollers, and the use of a doctor blade mechanism control the amount of ink transferred to the printing plate. The shape and volume of the cells are chosen to suit the anilox surface (chrome or ceramic), the doctoring system, the press capabilities, the printing substrate, and the image type (solid or halftone). Advances in anilox technology have resulted in laser-engraved ceramic anilox rollers offering tougher and longwearing rollers with greatly improved ink release characteristics compared to conventional mechanically engraved chrome roller technology.
Flexography prints can be made with a flexible printing plate that is wrapped around a rotating cylinder. The plate is usually made of natural or synthetic rubber or a photosensitive plastic material called photopolymer. It is usually attached to the plate cylinder with double-sided sticky tape. Flexography is a relief printing process, meaning that the image area on the printing plate is raised above the non-image area.
The image area receives the ink from the anilox roller, which is transferred to the print substrate when the latter is pressed with support of the impression cylinder against the printing plate. Flexography is a direct method, that is, the printing plate transfers the ink directly to the substrate.
Due to improved registration, a popular type of press is the CI press (central-impression) where printing units are arranged around a single central impression cylinder.
In general, the higher the speed of the press, the wider the press will be. When the press is wider and faster, the diameter of the anilox roller must be greater in order to prevent damage to the roller due to deflection and bending. A 50-inch (ca 127 cm) machine has a 6-inch (ca 15 cm) diameter anilox cylinder. The dwell time between the chamber and the ink transfer nip is shorter.
Linear speeds in excess of 1800 ft/min (ca 0.549 km/min) are considered high speed for printing flexible substrates, and presses with the capability of printing at a linear speed of 3300 ft/min (ca 1 km/min) are now appearing on the market.
The linear speed of 3300 ft/min (ca 1 km/min) is equal to a linear velocity of 35 miles per hour (ca 56.3 km/hr), and conventional plates and the double-sided sticky tape will eject from the press at this speed. In place of plates and double-sided sticky tape, direct laser engraved elastomer sleeves are used for printing at these velocities. The usual chambered doctor blade has a two-inch gap between the blades, and the dwell time for this distance at 3300 ft/min (ca 1 km/min) is less than the time of a high speed shutter on a 35 mm camera. In that interval, the air must be displaced from the cells of the anilox, ink must enter the cells, and the air must be cycled out from the chamber.
At linear speeds up to 2300 ft/min (ca 0.701 km/min), normal motors can be used; however, at linear speeds over 2300 ft/min water-cooled motors are preferred.
Many printers require inks and coatings to print at high speeds in order to improve the cost effectiveness of their operations. Flexographic printing linear speeds generally range up to 2000 ft/min (ca 0.609 km/min), and that speed can be expected to increase. At increasing linear speeds, for example greater than 1200 ft/min (ca 0.366 km/min), and especially 1800 ft/per minute (ca 0.549 km/min), the printability of the ink begins to deteriorate and print defects can be observed. This defect can be described as uniformly dispersed, irregularly shaped missed areas of printing. These defects are believed to result from the inability of the ink to wet out the surfaces of the printing blanket or plate or the substrate, or from the distinct mechanistic demands associated with a high speed printing press configuration as discussed in the above paragraphs.
Gravure printing is an example of intaglio printing. It uses a depressed or sunken surface for the image so that the image areas is generally honey comb shaped cells or wells that are etched or engraved into a printing cylinder. The unetched areas of the cylinder represent the non-image or unprinted areas. The cylinder rotates through an ink bath and excess ink is wiped off the cylinder by a flexible steel doctor blade. The ink remaining in the recessed cells forms the image by direct transfer to the substrate (paper or other material) as it passes between the plate cylinder and the impression cylinder.
Gravure inks are fluid inks with a very low viscosity that allows them to be drawn into the engraved cells in the cylinder then transferred onto the substrate. Flexographic and gravure inks are very similar and the basic constituents are essentially the same.
The transfer of ink to the substrate may be one of the most important factors affecting the quality of the final printed product. However, due to dynamics of linear high-speed presses, conventional inks used for slower speeds will breakdown at high speeds, creating print defects. Any print defect will negatively affect productivity and the inherent printing advantages of using linear high-speed presses.
Typical flexographic/gravure printing inks contain resins, solvents, colorants, and additives. The resins include rosin esters, polyamides, polyurethanes, nitrocellulose, and others. The solvents are often based on alcohols, acetates, glycol ethers, and possibly other solvent classes.
Suspensions form the backbone of several industrial materials such as coatings, inks, paints, ceramics and cosmetics. The control and improvement of the rheology and stability of such materials in chemical engineering processes have been a significant concern of scientists and engineers for decades. One significant feature of suspensions rheology is their viscoelastic properties. Viscoelastic properties of a fluid relates to the extent of the solid-like or liquid-like character of the fluid.
There are some articles in the literature studying the viscoelastic properties of paste inks. See, e.g., U.S. Pat. Nos. 7,267,055B2 and 6,602,333B2. However, the viscoelasticity of flexographic inks is rarely determined in the industry as the common assumption of flexo inks is not considering any viscoelasticity in such low viscosity inks. Rare occasions can be found on discussing the viscoelasticity water-based flexo inks and its relation with certain performance factors. One example is the work of Mai R., Pekarovicova A., Fleming III, P. D., Savarmand, S., and Chandorkar, O. V., “Ink Rheology and Press Performance of Water-based Flexographic Inks,” Flexo, 49-53 (October 2007). This work concludes that the more elastic the ink, the less dot gain.
Pinholing is one example of common runnability problems for the high-speed printing of the flexographic and rotogravure inks, and thus for the quality of the print derived therefrom. There is a need to reduce pinholing for the high-speeding printing of the flexographic and rotogravure inks.
The inventors of the present application discovered one factor relating to the pinholing defect on the surface of flexographic and rotogravure prints is the viscoelasticity of the ink. It is observed that a more elastic flexographic or rotogravure ink delays the formation of pin-holes on the final print to higher printing speeds, and a postulated mechanism for this relates to its action in relaxing nonuniformities caused by the ink transfer and film-split.